1. Field of the Invention
The present invention relates to motion picture encoding method and device.
2. Description of the Related Art
In international standards of motion picture encoding as in MPEG-2, H.264, either one of a frame-based prediction mode and a field-based prediction mode can be selected as a motion compensation prediction mode for use in encoding an interlace image. In the frame-based prediction mode, a frame is used as a unit (referred to as a picture in MPEG) of encoding. In the field-based prediction mode, a field is used as a unit of encoding. When the frame is used as the unit of encoding, a frame structure is referred to. When the field is used as the unit of encoding, a field structure is referred to.
In a case where an image has little motion, there is strong correlation between the fields. Therefore, it is more advantageous to use the frame-based prediction mode in motion compensation prediction. On the other hand, in a case where an image has an intense motion, since the correlation between the fields is weak, it is more advantageous to use the field-based prediction mode in the motion compensation prediction. To perform highly efficient encoding, it is necessary to appropriately select the motion compensation prediction mode. In Jpn. Pat. Appln. KOKAI Publication No. 2004-266489, a method is disclosed to select a mode having a larger vertical correlation coefficient of an image signal from the frame-based prediction mode and the field-based prediction mode. In MPEG Software Simulation Group, “Test Model 5 (aka TM5)” <URL:http://www.mpeg.org/MPEG/MSSG/tm 5/5>, a method is disclosed to select a mode having a smaller sum of absolute values of predictive residual signals from the frame-based prediction mode and the field-based prediction mode.
A method of using the vertical correlation coefficient of the image signal in evaluating the motion compensation prediction mode is effective for in-frame coding to directly encode the image signal, but it is difficult to precisely select the prediction mode in inter-frame coding to encode the predictive residual signal.
On the other hand, in the method of using the predictive residual signal in the evaluation of the motion compensation prediction mode, it is possible to precisely select the prediction mode even in the inter-frame coding. However, since the mode is evaluated by the sum of the absolute values of the predictive residual signal, any similarity between lines is not evaluated. In other words, any high-pass component of the predictive residual signal, which influences an encoding efficiency during orthogonal transformation encoding of the predictive residual signal, is not considered. Therefore, a preferable prediction mode is not necessarily selected.
A technology referred to as hierarchical searching is known for the purpose of reducing an amount of calculation required for detecting a motion vector, which occupies a large part of processing during motion picture encoding. In the hierarchical searching, first a coarse motion vector is searched, and subsequently a fine motion vector is searched. According to the hierarchical searching, in many cases, no difference is observed between sums of absolute values of predictive residual signals in a frame structure and a field structure at a time when the coarse motion vector is searched. Therefore, it is necessary to perform the fine motion vector searching in both the frame structure and the field structure, and the amount of calculation increases.